Thermoplastic resin microporous membranes are used widely as separation membranes, selective transmission membranes, isolation membranes, and the like for substances. For example, the usage includes battery separators used in lithium ion secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, or polymer batteries, separators for electric double layer capacitors, and the like.
In particular, a polyethylene microporous membrane which exhibits ion permeability due to electrolytic solution impregnation, excellent electrical insulating properties, and a pore blocking function for blocking an electrical current to prevent an excessive temperature increase at a temperature of approximately 120 to 150° C. at the time of an abnormal temperature increase in a battery, is suitably used as a lithium ion secondary battery separator. However, if the internal temperature of the battery continues to increase even after pore blocking for some reason, the polyethylene microporous membrane may be punctured due to the shrinkage of the membrane. This phenomenon is not limited to polyethylene microporous membranes. Even in the case of a microporous membrane using another thermoplastic resin, this phenomenon cannot be avoided at a temperature equal to or above the melting point of the resin.
In particular, separators for lithium-ion batteries greatly affect battery characteristics, battery productivity and battery safety, and require mechanical properties, heat resistance, permeability, dimensional stability, shutdown characteristics, membrane melt-puncture characteristics (melt-down characteristics) and the like. In addition, they require improved adhesion to an electrode material for improvement in cycle characteristics of batteries and improved wettability toward electrolytic solution for productivity improvement. The notion of enhancing these functions by providing a porous layer on a microporous membrane has been investigated in the past.
Note that a porous layer described in this specification refers to a layer obtained by a wet coating method.
In Example 5 of Patent Document 1, a multilayer porous membrane having a total membrane thickness of 24 μm (coating thickness: 4 μm) is obtained by using a gravure coater to apply an aqueous solution in which titania particles and polyvinyl alcohol are uniformly dispersed to a polyethylene microporous membrane precursor obtained by a simultaneous biaxial stretching method, and then drying the product at 60° C. to remove water.
In Example 3 of Patent Document 2, a multilayer porous membrane having a total membrane thickness of 19 μm (coating thickness: 3 μm) is obtained by using a bar coater to apply an aqueous solution in which titania particles and polyvinyl alcohol are uniformly dispersed to a polyolefin microporous membrane obtained by a simultaneous biaxial stretching method, and then drying the product at 60° C. to remove water.
In Example 1 of Patent Document 3, a multilayer porous membrane having a total membrane thickness of 20 μm (coating thickness: 4 μm) is obtained by using a gravure coater to apply an aqueous solution in which aluminum particles and polyvinyl alcohol are uniformly dispersed to a polyolefin microporous membrane obtained by a simultaneous biaxial stretching method, and then drying the product at 60° C. to remove water.
In Example 6 in Patent Document 4, a polyethylene microporous membrane obtained by a sequential biaxial stretching method is passed through between Meyer bars, on which an appropriate amount of a coating solution containing meta-type wholly aromatic polyamide, an alumina particle, dimethylacetamide (DMAc) and tripropylene glycol (TPG) is provided, then subjected to coagulation, water washing, and drying steps, to obtain a non-water-based separator for a rechargeable battery, in which a heat-resistant porous layer is formed.
In Patent Document 5, a polyethylene microporous membrane obtained by a sequential biaxial stretching method is passed through between facing Meyer bars, on which an appropriate amount of a coating solution containing meta-type wholly aromatic polyamide, aluminum hydroxide. DMAc and TPG is provided, then subjected to coagulation, water washing, and drying steps, to obtain a non-water-based separator for a rechargeable battery, in which a heat-resistant porous layer is formed.
In Patent Document 6, a polyethylene microporous membrane obtained by a sequential biaxial stretching method is passed through between facing Meyer bars, on which an appropriate amount of a coating solution containing polymetaphenylene isophthalamide and aluminum hydroxide, DMAc and TPG is provided, then subjected to coagulation, water washing, and drying steps, to obtain a non-water-based separator for a rechargeable battery, in which a heat-resistant porous layer is formed.
In Patent Document 7, a laminated porous film is obtained by combining a so-called sequential biaxial stretching method and an in-line coating method, wherein a non-porous membrane-like material with a three-layer structure having a layer including polypropylene containing a β-crystal nucleating agent in the outer layer is stretched in the longitudinal direction using a longitudinal stretching device, and after an aqueous dispersion containing alumina crystals and polyvinyl alcohol is then applied using a Meyer bar, the product is stretched in the transverse direction to twice the width and subjected to heat setting/relaxation treatment.
In Patent Document 8, a separation membrane obtained with a sequential biaxial stretching method using a stretching method of setting the angle at which an object to be stretched and a stretching roller make contact to at least a certain angle in a longitudinal direction stretching machine consisting of four stretching rollers is disclosed.
In Patent Document 9, a method of manufacturing a microporous membrane by stretching a laminate comprising a layer which includes low-melting point polymer and a layer which does not include low-melting point polymer is disclosed.